Increased seizure threshold in mice lacking aquaporin-4 water channels

Discovery of the aquaporins and development of the field

The study of water transport began long before the molecular identification of water channels with studies of water-permeable tissues. The discovery of the first aquaporin, AQP1, occurred during experiments focused on the identity of the Rh blood group antigens. Since then the field has expanded dramatically to study aquaporins in all types of organisms. In mammals, some of the aquaporins transport only water. However, there are some family members that collectively transport a diverse set of solutes. The aquaporins can be regulated by factors that affect channel permeability or subcellular localization. An extensive set of studies examines the physiological role of many of the mammalian aquaporins. However, much is still to be discovered about the physiological role of this membrane protein family.

Knock-out models reveal new aquaporin functions

Knockout mice have been informative in the discovery of unexpected biological functions of aquaporins. Knockout mice have confirmed the predicted roles of aquaporins in transepithelial fluid transport, as in the urinary concentrating mechanism and glandular fluid secretion. A less obvious, though predictable role of aquaporins is in tissue swelling under stress, as in the brain in stroke, tumor and infection. Phenotype analysis of aquaporin knockout mice has revealed several unexpected cellular roles of aquaporins whose mechanisms are being elucidated. Aquaporins facilitate cell migration, as seen in aquaporin-dependent tumor angiogenesis and tumor metastasis, by a mechanism that may involve facilitated water transport in lamellipodia of migrating cells. The ' aquaglyceroporins', aquaporins that transport both glycerol and water, regulate glycerol content in epidermis, fat and other tissues, and lead to a multiplicity of interesting consequences of gene disruption including dry skin, resistance to skin carcinogenesis, impaired cell proliferation and altered fat metabolism. An even more surprising role of a mammalian aquaporin is in neural signal transduction in the central nervous system. The many roles of aquaporins might be exploited for clinical benefit by modulation of aquaporin expression/function - as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.

AQP4 knockout impairs proliferation, migration and neuronal differentiation of adult neural stem cells

Aquaporin-4 (AQP4), a key molecule for maintaining water and ion homeostasis in the central nervous system, is expressed in adult neural stem cells (ANSCs) as well as astrocytes. However, little is known about the functions of AQP4 in the ANSCs in vitro. Here we show that AQP4 knockout inhibits the proliferation, survival, migration and neuronal differentiation of ANSCs derived from the subventricular zone of adult mice. Flow cytometric cell cycle analysis revealed that AQP4 knockout increased the basal apoptosis and induced a G2-M arrest in ANSCs. Using Fluo-3 Ca2+ imaging, we show that AQP4 knockout alters the spontaneous Ca2+ oscillations by frequency enhancement and amplitude suppression, and suppresses KCl-induced Ca2+ influx. AQP4 knockout downregulated the expression of connexin43 and the L-type voltage-gated Ca2+ channel CaV1.2 subtype in ANSCs. Together, these findings suggest that AQP4 plays a crucial role in regulating the proliferation, migration and differentiation of ANSCs, and this function of AQP4 is probably mediated by its action on intracellular Ca2+ dynamics.

Diffusion in brain extracellular space

Diffusion in the extracellular space (ECS) of the brain is constrained by the volume fraction and the tortuosity and a modified diffusion equation represents the transport behavior of many molecules in the brain. Deviations from the equation reveal loss of molecules across the blood-brain barrier, through cellular uptake, binding, or other mechanisms. Early diffusion measurements used radiolabeled sucrose and other tracers. Presently, the real-time iontophoresis (RTI) method is employed for small ions and the integrative optical imaging (IOI) method for fluorescent macromolecules, including dextrans or proteins. Theoretical models and simulations of the ECS have explored the influence of ECS geometry, effects of dead-space microdomains, extracellular matrix, and interaction of macromolecules with ECS channels. Extensive experimental studies with the RTI method employing the cation tetramethylammonium (TMA) in normal brain tissue show that the volume fraction of the ECS typically is approximately 20% and the tortuosity is approximately 1.6 (i.e., free diffusion coefficient of TMA is reduced by 2.6), although there are regional variations. These parameters change during development and aging. Diffusion properties have been characterized in several interventions, including brain stimulation, osmotic challenge, and knockout of extracellular matrix components. Measurements have also been made during ischemia, in models of Alzheimer's and Parkinson's diseases, and in human gliomas. Overall, these studies improve our conception of ECS structure and the roles of glia and extracellular matrix in modulating the ECS microenvironment. Knowledge of ECS diffusion properties is valuable in contexts ranging from understanding extrasynaptic volume transmission to the development of paradigms for drug delivery to the brain.

Lack of aquaporin-4 water transport inhibition by antiepileptics and arylsulfonamides

Inhibitors of brain glial water channel aquaporin-4 (AQP4) are of potential clinical utility, as they are predicted to modulate brain edema, neuroexcitation and glial scarring. Recently, Huber et al. (Bioorg. Med. Chem.2007, 17, 1270-1273; in press) reported that a series of arylsulfonamides, antiepileptics, and related small molecules strongly inhibited AQP4 water transport with IC(50)s down to 1 microM. We retested the compounds with greatest reported potencies, including acetylsulfanilamide, acetazolamide, 6-ethoxy-benzothiazole-2-sulfonamide, topiramate, zonisamide, phenytoin, lamotrigine, and sumatriptan, in AQP4-transfected mammalian cells and primary cultures of brain glial cells, using several sensitive assays of osmotic water permeability. Contrary to the findings of Huber et al., in our studies we found no significant inhibition of AQP4 water permeability by any of the compounds at concentrations up to 100 microM.

Hypersensitivity of aquaporin 4-deficient mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine and astrocytic modulation

Aquaporin 4 (AQP4) is a predominant water channel protein in mammalian brains, which is localized in the astrocyte plasma membrane. AQP4 has gained much attraction due to its involvement in the physiopathology of cerebral disorders including stroke, tumor, infection, hydrocephalus, epilepsy, and traumatic brain injury. But there is almost no evidence whether abnormal AQP4 levels are associated with degenerative diseases, such as Parkinson's disease (PD). In our studies, we established PD animal models by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine to test the hypothesis that abnormal AQP4 expression is involved in the pathophysiology of this disease. We show that mutant mice lacking AQP4 were significantly more prone to MPTP-induced neurotoxicity than their wild-type littermates. Furthermore, after administration of MPTP, astroglial proliferation and GDNF protein synthesis were inhibited by AQP4 deficiency. This study demonstrates that AQP4 is important in the MPTP neurotoxic process and indicates that the therapeutic strategy targeted to astrocytic modulation with AQP4 may offer a great potential for the development of new treatment for PD.

Aquaporin-4-deficient mice have increased extracellular space without tortuosity change

Aquaporin-4 (AQP4) is the major water channel expressed at fluid-tissue barriers throughout the brain and plays a crucial role in cerebral water balance. To assess whether these channels influence brain extracellular space (ECS) under resting physiological conditions, we used the established real-time iontophoresis method with tetramethylammonium (TMA(+)) to measure three diffusion parameters: ECS volume fraction (alpha), tortuosity (lambda), and TMA(+) loss (k'). In vivo measurements were performed in the somatosensory cortex of AQP4-deficient (AQP4(-/-)) mice and wild-type controls with matched age. Mice lacking AQP4 showed a 28% increase in alpha (0.23 +/- 0.007 vs 0.18 +/- 0.003) with no differences in lambda (1.62 +/- 0.04 vs 1.61 +/- 0.02) and k' (0.0045 +/- 0.0001 vs 0.0031 +/- 0.0009 s(-1)). Additional recordings in brain slices showed similarly elevated alpha in AQP4(-/-) mice, and no differences in lambda and k' between the two genotypes. This is the first direct comparison of ECS properties in adult mice lacking AQP4 water channels with wild-type animals and demonstrates a significant enlargement of the volume fraction but no difference in hindrance to TMA(+) diffusion, expressed as tortuosity. These findings provide direct evidence for involvement of AQP4 in modulation of the ECS volume fraction and provide a basis for future modeling of water and ion transport in the CNS.

Novel variants in human Aquaporin-4 reduce cellular water permeability

Cerebral edema contributes significantly to morbidity and mortality after brain injury and stroke. Aquaporin-4 (AQP4), a water channel expressed in astrocytes, plays a key role in brain water homeostasis. Genetic variants in other aquaporin family members have been associated with disease phenotypes. However, in human AQP4, only one non-synonymous single-nucleotide polymorphism (nsSNP) has been reported, with no characterization of protein function or disease phenotype. We analyzed DNA from an ethnically diverse cohort of 188 individuals to identify novel AQP4 variants. AQP4 variants were constructed by site-directed mutagenesis and expressed in cells. Water permeability assays in the cells were used to measure protein function. We identified 24 variants in AQP4 including four novel nsSNPs (I128T, D184E, I205L and M224T). We did not observe the previously documented M278T in our sample. The nsSNPs found were rare ( approximately 1-2% allele frequency) and heterozygous. Computational analysis predicted reduced function mutations. Protein expression and membrane localization were similar for reference AQP4 and the five AQP4 mutants. Cellular assays confirmed that four variant AQP4 channels reduced normalized water permeability to between 26 and 48% of the reference (P < 0.001), while the M278T mutation increased normalized water permeability (P < 0.001). We identified multiple novel AQP4 SNPs and showed that four nsSNPs reduced water permeability. The previously reported M278T mutation resulted in gain of function. Our experiments provide insight into the function of the AQP4 protein. These nsSNPs may have clinical implications for patients with cerebral edema and related disorders.

Impaired olfaction in mice lacking aquaporin-4 water channels

Aquaporin-4 (AQP4) is a water-selective transport protein expressed in glial cells throughout the central nervous system. AQP4 deletion in mice produces alterations in several neuroexcitation phenomena, including hearing, vision, epilepsy, and cortical spreading depression. Here, we report defective olfaction and electroolfactogram responses in AQP4-null mice. Immunofluorescence indicated strong AQP4 expression in supportive cells of the nasal olfactory epithelium. The olfactory epithelium in AQP4-null mice had identical appearance, but did not express AQP4, and had approximately 12-fold reduced osmotic water permeability. Behavioral analysis showed greatly impaired olfaction in AQP4-null mice, with latency times of 17 +/- 0.7 vs. 55 +/- 5 s in wild-type vs. AQP4-null mice in a buried food pellet test, which was confirmed using an olfactory maze test. Electroolfactogram voltage responses to multiple odorants were reduced in AQP4-null mice, with maximal responses to triethylamine of 0.80 +/- 0.07 vs. 0.28 +/- 0.03 mV. Similar olfaction and electroolfactogram defects were found in outbred (CD1) and inbred (C57/bl6) mouse genetic backgrounds. Our results establish AQP4 as a novel determinant of olfaction, the deficiency of which probably impairs extracellular space K(+) buffering in the olfactory epithelium.

Functions of aquaporins in the eye

The aquaporins (AQPs) are integral membrane proteins whose main function is to transport water across cell membranes in response to osmotic gradients. At the ocular surface, AQP1 is expressed in corneal endothelium, AQP3 and AQP5 in corneal epithelium, and AQP3 in conjunctival epithelium. AQPs are also expressed in lens fiber cells (AQP0), lens epithelium (AQP1), ciliary epithelium (AQP1, AQP4) and retinal Müller cells (AQP4). Mutations in AQP0 produce congenital cataracts in humans. Analysis of knockout mice lacking individual AQPs suggests their involvement in maintenance of corneal and lens transparency, corneal epithelial repair, intraocular pressure (IOP) regulation, retinal signal transduction and retinal swelling following injury. The mouse phenotype findings implicate AQPs as potential drug targets for therapy of elevated IOP and ocular disorders involving the cornea, lens and retina. However, much research remains in defining cell-level mechanisms for the ocular AQP functions, in establishing the relevance to human eye disease of conclusions from knockout mice, and in developing AQP-modulating drugs.

Mammalian aquaporins: diverse physiological roles and potential clinical significance

Aquaporins have multiple distinct roles in mammalian physiology. Phenotype analysis of aquaporin-knockout mice has confirmed the predicted role of aquaporins in osmotically driven transepithelial fluid transport, as occurs in the urinary concentrating mechanism and glandular fluid secretion. Aquaporins also facilitate water movement into and out of the brain in various pathologies such as stroke, tumour, infection and hydrocephalus. A major, unexpected cellular role of aquaporins was revealed by analysis of knockout mice: aquaporins facilitate cell migration, as occurs in angiogenesis, tumour metastasis, wound healing, and glial scar formation. Another unexpected role of aquaporins is in neural function - in sensory signalling and seizure activity. The water-transporting function of aquaporins is likely responsible for these roles. A subset of aquaporins that transport both water and glycerol, the 'aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues. Mice lacking various aquaglyceroporins have several interesting phenotypes, including dry skin, resistance to skin carcinogenesis, impaired cell proliferation, and altered fat metabolism. The various roles of aquaporins might be exploited clinically by development of drugs to alter aquaporin expression or function, which could serve as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.

Identification of aquaporin 4 inhibitors using in vitro and in silico methods

The in vitro inhibitory effects and in silico docking energies of 18 compounds with respect to aquaporin 4 (AQP4) were investigated. More than half of the compounds tested showed inhibitory activity in the in vitro functional assay and included the 5-HT(1B/1D) agonists sumatriptan, and rizatriptan. Moreover, the observed inhibitory activity of the compounds used in this study at 20 microM showed a strong correlation with their in silico docking energies, r(2)=0.64, which was consistent with that found in previous studies. The AQP4 inhibitory IC(50) values of three compounds, 2-(nicotinamido)-1,3,4-thiadiazole, sumatriptan and rizatriptan, were subsequently found to be 3, 11, and 2 microM, respectively.

Water movements in the brain: role of aquaporins

About 80% of the brain is water. This review discusses the importance of the three brain water-channel proteins (AQP1, AQP4, AQP9) in brain physiology. AQP1 is expressed in the choroid plexus and participates in forming cerebrospinal fluid. AQP4, found in astrocyte foot processes, glia limitans and ependyma, facilitates water movement into and out of the brain, accelerates astrocyte migration and alters neuronal activity. Recently, AQP4 autoantibodies were discovered in patients with neuromyelitis optica, a demyelinating disease, and are now being used to diagnose this condition. AQP9 is present in some glia and neurons, but its function is unclear. Finally, we discuss how the discovery of AQP activators and inhibitors will be the next major step in this field.

Aquaporin 1 and aquaporin 4 overexpression in bovine spongiform encephalopathy in a transgenic murine model and in cattle field cases

Aquaporins (AQP) are a family of transmembrane proteins that act as water selective channels. AQP1 and AQP4 are widely expressed in the central nervous system where they play several roles. Overexpression of AQP has been reported in some human and animal transmissible spongiform encephalopathies, but information is scanty about their distribution in the central nervous system in bovine spongiform encephalopathy (BSE). Double immunohistochemistry for AQP1, AQP4 and GFAP was developed in a transgenic mouse line overexpressing the bovine cellular prion protein (BoTg110), intracerebrally infected with cattle BSE. Western blot for AQP1 and AQP4, and immunohistochemistry for both AQP and GFAP were carried out in cases of BSE-diagnosed cattle as part of surveillance plan in Catalonia (Spain). A marked increase in AQP1 and AQP4 was observed in mice at the terminal stage of the disease, when they had a wide range of clinical signs, whereas no increase could be observed in the early stage before the onset of the clinical signs. In cattle which did not show evidence of clinical signs, both AQP already showed a great increase. The AQP overexpression correlated with GFAP-immunoreactive astrocytes and PrPres deposition in both cases. The results of this study suggest that AQP overexpression in glial cells could lead to an imbalance in water and ion homeostasis which could contribute to triggering the typical histopathological changes of BSE.

Aquaporin-4 independent Kir4.1 K+ channel function in brain glial cells

Functional interaction of glial water channel aquaporin-4 (AQP4) and inwardly rectifying K+ channel Kir4.1 has been suggested from their apparent colocalization and biochemical interaction, and from the slowed glial cell K+ uptake in AQP4-deficient brain. Here, we report multiple lines of evidence against functionally significant AQP4-Kir4.1 interactions. Whole-cell patch-clamp of freshly isolated glial cells from brains of wild-type and AQP4 null mice showed no significant differences in membrane potential, barium-sensitive Kir4.1 K+ current or current-voltage curves. Single-channel patch-clamp showed no differences in Kir4.1 unitary conductance, voltage-dependent open probability or current-voltage relationship. Also, Kir4.1 protein expression and distribution were similar in wild-type and AQP4 null mouse brain and in the freshly isolated glial cells. Functional inhibition of Kir4.1 by barium or RNAi knock-down in primary glial cell cultures from mouse brain did not significantly alter AQP4 water permeability, as assayed by calcein fluorescence quenching following osmotic challenge. These studies provide direct evidence against functionally significant AQP4-Kir4.1 interactions in mouse glial cells, indicating the need to identify new mechanism(s) to account for altered seizure dynamics and extracellular space K+ buffering in AQP4 deficiency.

Evidence against Functional Interaction between Aquaporin-4 Water Channels and Kir4.1 Potassium Channels in Retinal Müller Cells

Indirect evidence suggests that the Müller/glial cell water channel aquaporin-4 (AQP4) modulates K(+) channel function of the closely associated Kir4.1 protein. We used patch clamp to compare Kir4.1 K(+) channel function in freshly isolated Müller cells from retinas of wild-type (+/+) and AQP4 knock-out (-/-) mice. Immunocytochemistry showed a comparable Kir4.1 protein expression pattern in Müller cells from +/+ and -/- retinas, with greatest expression at their end feet. Osmotic water permeability was >4-fold reduced in -/- than in +/+ Müller cells. Resting membrane potential did not differ significantly in +/+ versus -/- Müller cells (-64 +/- 1 versus -64 +/- 1 mV, S.E., n = 24). Whole-cell K(+) currents recorded with a micropipette inserted into the cell soma were Ba(2+)-sensitive and showed no significant differences in magnitude in +/+ versus -/- Müller cells (1.3 +/- 0.1 versus 1.2 +/- 0.1 nA at -160 mV) or in inwardly rectifying current-voltage relationships. Spatially resolved K(+) currents generated by pulsed K(+) injections along Müller cell bodies were also comparable in +/+ versus -/- Müller cells. Single-channel cell-attached patch clamp showed comparable unitary conductance, current-voltage data, and open probability in +/+ versus -/- Müller cells. Thus, contrary to the generally accepted view, our results provide direct evidence against functionally significant AQP4 modulation of Müller cell Kir4.1 K(+) channel function.

Alterations of striatal neurotransmitter release in aquaporin-4 deficient mice: An in vivo microdialysis study

Our previous investigation has demonstrated that the lack of aquaporin-4 (AQP4) expressions in mice is paralleled by sex- and region-specific abnormalities in neurotransmission. In the present study, we examined the effect of AQP4 deficiency on release of neurotransmitters in mouse striatum using in vivo microdialysis after high K(+) stimulus. The results showed that neurotransmitter releases under the basal and K(+)-stimulated conditions in the striatum of wildtype mice were similar to AQP4 knockout mice, except for taurine, when measured at 24h after microdialysis surgery. However, the basal extracellular levels of dopamine and its metabolites were significantly increased in knockout mice, followed by reduced or no response to depolarizing stimuli when measured at 7 d after surgery. In addition, it was found that there were higher responses of amino acids to high K(+) stimulus in knockout mice. This experiment provides the in vivo evidence that AQP4 participates in the regulation of neurotransmitter release induced by depolarizing stimuli.

Human astrocytes express aquaporin-1 and aquaporin-4 in vitro and in vivo

Aquaporins (AQP) constitute an evolutionarily conserved family of integral membrane water transport channel proteins. Previous studies indicate that AQP1 is expressed exclusively in the choroid plexus epithelium, while AQP4 is localized on the vascular foot of astrocytes in the central nervous system (CNS) under physiological conditions. To investigate a role of AQP in the pathophysiology of neurological diseases involving astrogliosis we studied the expression of AQP1 and AQP4 in cultured human astrocytes and brain tissues of multiple sclerosis (MS), cerebral infarction and control cases. By reverse transcriptasepolymerase chain reaction and western blot analysis, cultured human astrocytes co-expressed both AQP1 and AQP4 mRNA and proteins, where AQP4 levels were elevated by exposure to interferon-gamma but neither by tumor necrosis factor-alpha nor interleukin-1beta, whereas AQP1 levels were unaffected by any of the cytokines examined. By western blot analysis, AQP1 and AQP4 proteins were detected in the brain homogenates of the MS and non-MS cases, where both levels were correlated with those of glial fibrillary acid protein. By immunohistochemistry, astrocytes with highly branched processes surrounding blood vessels, along with glial scar, expressed intensely AQP1 and AQP4 in MS and ischemic brain lesions, whereas neither macrophages, neurons nor oligodendrocyte cell bodies were immunopositive. These immunohistochemical results indicate that the expression not only of AQP4 but also of AQP1 was enhanced in MS and ischemic brain lesions located predominantly in astrocytes, suggesting a pivotal role of astrocytic AQP in the maintenance of water homeostasis in the CNS under pathological conditions.

Identification of arylsulfonamides as Aquaporin 4 inhibitors

Carbonic anhydrase inhibitors AZA, EZA, and 4-acetamidobenzsulfonamide were found to inhibit human AQP4-M23 mediated water transport by 80%, 68%, and 23%, respectively, at 20 microM in an in vitro functional assay. AZA was found to have an IC50 against AQP4 of 0.9 microM. Phloretin was inactive under the same conditions.

Aquaporin 4 regulates the effects of ovarian hormones on monoamine neurotransmission

Aquaporin 4 (AQP4) is the predominant water channels in the brain of mammals. Our previous study has reported that AQP4 knockout induced sex-specific alterations in neurotransmission, indicating that AQP4 might regulate the interaction between sex hormones and neurotransmission. In the present study, we found that AQP4 knockout decreased the concentrations of estrogen and progestogen. Further study showed that exogenous estrogen decreased DA and 5-HT in cortex, reduced DA and 5-HT in striatum, but increased 5-HT in hippocampus in AQP4+/+ male mice. However, in AQP4-/- male mice, exogenous estrogen almost did not alter the levels of neurotransmitters except for decreasing DA in cortex. In female mice, ovariectomy decreased DA in the striatum of AQP4+/+ mice, but did not alter the levels of DA in AQP4-/- mice. These findings reveal for the first time that AQP4 regulates not only water and ion homeostasis but also the functions of ovarian hormone and neurotransmitter.

Aquaporin-4 and brain edema

Aquaporin-4 (AQP4) is a water-channel protein expressed strongly in the brain, predominantly in astrocyte foot processes at the borders between the brain parenchyma and major fluid compartments, including cerebrospinal fluid (CSF) and blood. This distribution suggests that AQP4 controls water fluxes into and out of the brain parenchyma. Experiments using AQP4-null mice provide strong evidence for AQP4 involvement in cerebral water balance. AQP4-null mice are protected from cellular (cytotoxic) brain edema produced by water intoxication, brain ischemia, or meningitis. However, AQP4 deletion aggravates vasogenic (fluid leak) brain edema produced by tumor, cortical freeze, intraparenchymal fluid infusion, or brain abscess. In cytotoxic edema, AQP4 deletion slows the rate of water entry into brain, whereas in vasogenic edema, AQP4 deletion reduces the rate of water outflow from brain parenchyma. AQP4 deletion also worsens obstructive hydrocephalus. Recently, AQP4 was also found to play a major role in processes unrelated to brain edema, including astrocyte migration and neuronal excitability. These findings suggest that modulation of AQP4 expression or function may be beneficial in several cerebral disorders, including hyponatremic brain edema, hydrocephalus, stroke, tumor, infection, epilepsy, and traumatic brain injury.

Expression pattern of the water channel aquaporin-4 in human gliomas is associated with blood–brain barrier disturbance but not with patient survival

Aquaporin-4 (AQP4), the most prominent CNS water channel, is restricted to the glia limitans and astrocytic endfeet. We previously showed the loss of spatial AQP4 expression in glioblastomas and a redistribution across the cell surface. However, opposing AQP4 functions have been described: protective in vasogenic but detrimental in cytotoxic brain edema. Thus, specific AQP4 induction to prevent or reduce vasogenic edema is suggested. To elucidate the AQP4 role in brain tumors, we investigated 189 WHO grade I-IV gliomas by immunohistochemistry and the prognostic significance for patients' survival. In gliomas, a remarkable de novo AQP4 redistribution was observed in comparison with normal CNS tissue. Surprisingly, the highest membraneous staining levels were seen in pilocytic astrocytomas WHO grade I and grade IV glioblastomas, both significantly higher than in WHO grade II astrocytomas. AQP4 up-regulation was associated with brain edema formation; however, no association between survival and WHO grade-dependent AQP4 expression was seen. Hence, AQP4 redistribution may go along with other tumor properties, such as vascular proliferation and resulting blood-brain barrier disturbance, features usually prominent in pilocytic astrocytomas WHO I and glioblastomas WHO grade IV. In summary, our findings question the protective role of AQP4 in vasogenic brain edema. Although AQP4 was associated with brain edema formation, one has to question the suitability of AQP4 induction as a promising approach in vasogenic brain edema prevention and treatment. In addition, our results provide unexpectedly high AQP4 levels in pilocytic astrocytomas and present AQP4 as tumor progression marker in WHO grade II-IV astrocytomas.

Greatly impaired migration of implanted aquaporin-4-deficient astroglial cells in mouse brain toward a site of injury

We reported previously that astroglia cultured from aquaporin-4-deficient (AQP4-/-) mice migrate more slowly in vitro than those from wild-type (AQP4+/+) mice (J. Cell Sci. 2005;118, 5691-5698). Here, we investigate the migration of fluorescently labeled AQP4+/+ and AQP4-/- astroglia after implantation into mouse brains in which directional movement was stimulated by a planar stab wound 3 mm away from the axis of the injection needle. Two days after cell injection we determined the location, elongation ratio, and orientation of labeled cells. Migration of AQP4+/+ but not AQP4-/- cells toward the stab was greater than away from the stab. AQP4+/+ astroglia moved on average 1.5 mm toward the stab compared with 0.6 mm for AQP4-/- cells. More than 25% of the migrating AQP4+/+ cells but <3% of AQP4-/- cells appeared elongated (axial ratio>2.5). In transwell assays, AQP4+/+ astroglia migrated faster than AQP4-/- cells in a manner dependent on pore size. At 8 h, approximately 50% of AQP4+/+ cells migrated through 8-microm diameter pores, whereas equivalent migration of AQP4-/- cells was found for 12-microm diameter pores. These results provide in vivo evidence for AQP4-dependent astroglial migration and suggest that modulation of AQP4 expression or function might alter glial scarring.

Aquaporin-4 deficiency down-regulates glutamate uptake and GLT-1 expression in astrocytes

The role of aquaporin-4 in water transport has been extensively investigated, while little information exists regarding its contribution to astrocytic functions such as the action to glutamatergic transmission. Since aquaporin-4 has been detected widely co-localized with glutamate transporter 1 (GLT-1) and glutamate transporters also present water transport properties, we investigated the regulative role of aquporin-4 on glutamate transporter using primary cultured astrocytes from aquaporin-4 knockout (AQP4(-/-)) mice. It was demonstrated that lack of aquaporin-4 down-regulated astrocytic expression of GLT-1 but not of glutamate/aspartate transporter (GLAST). The result from [(3)H]D,L-glutamate uptake analysis showed a lower uptake capability in AQP4(-/-) astrocytes. Furthermore, MTT and LDH assays indicated less cellular toxicity induced by excessive glutamate in AQP4(-/-) genotype. These findings provide direct evidences for the first time that aquaporin-4 plays an important role in the function of glutamate transporters. And the present study will improve our understanding of aquaporin-4-glutamanergic biology.

Function of aquaporins in female and male reproductive systems

The flow of water and some other small molecules across cell membranes is important in many of the processes underlying reproduction. The fluid movement is strongly associated with the presence of aquaporins (AQPs) in the female and male reproductive systems. It has been suggested that AQPs mediate water movement into the antral follicle and play important roles in follicle development. AQPs are known to be involved in the early stage of spermatogenesis, in the secretion of tubule liquid and in the concentration and storage of spermatozoa. Fluid reabsorption in some regions of the male reproductive tract is under steroid hormone control and could be mediated by various AQPs. Also AQPs take part in the processes of fertilization, blastocyst formation (as the pathway for transtrophoectodermal water movement during cavitation) and implantation. Alterations in the expression and function or regulation of AQPs have already been demonstrated in disorders of the male reproductive system, such as abnormal sperm motility, the abnormal epididymis and infertility seen in cystic fibrosis, and varicocele. This article extensively reviews the distribution of AQPs in mammalian reproductive tissues and discusses their possible physiological and pathophysiological roles.

Novel roles of aquaporins revealed by phenotype analysis of knockout mice

The aquaporins (AQPs) are small integral membrane proteins that transport water and in some cases small solutes such as glycerol. Physiological roles of the ten or more mammalian AQPs have been proposed based on their expression in epithelial, endothelial and other tissues, their regulation, and in some cases the existence of humans with AQP mutation. Here, the role of AQPs in mammalian physiology is reviewed, based on phenotype analysis of transgenic mouse models of AQP deletion/mutation. Phenotype studies support the predicted roles of AQPs in kidney tubule and microvessel fluid transport for urinary concentrating function, and in fluid-secreting glandular epithelia. The phenotype studies have also shown unexpected roles of AQPs in brain and corneal swelling, in neural signal transduction, in regulation of intracranial and intraocular pressure, and in tumor angiogenesis and cell migration. The water/glycerol-transporting AQPs were found to play unexpected roles in skin hydration and in fat metabolism. However, many phenotype studies were negative, such as normal airway/lung and skeletal muscle function, despite AQP expression, indicating that tissue-specific AQP expression does not indicate physiological significance. The mouse phenotype data suggest that modulators of AQP expression/function may have such wide-ranging clinical applications as diuretics and in the treatment of brain swelling, glaucoma, epilepsy, obesity, and cancer.

Increased seizure duration and slowed potassium kinetics in mice lacking aquaporin-4 water channels

The glial water channel aquaporin-4 (AQP4) has been hypothesized to modulate water and potassium fluxes associated with neuronal activity. In this study, we examined the seizure phenotype of AQP4 -/- mice using in vivo electrical stimulation and electroencephalographic (EEG) recording. AQP4 -/- mice were found to have dramatically prolonged stimulation-evoked seizures after hippocampal stimulation compared to wild-type controls (33 +/- 2 s vs. 13 +/- 2 s). In addition, AQP4 -/- mice were found to have a higher seizure threshold (167 +/- 17 microA vs. 114 +/- 10 microA). To assess a potential effect of AQP4 on potassium kinetics, we used in vivo recording with potassium-sensitive microelectrodes after direct cortical stimulation. Although there was no significant difference in baseline or peak [K(+)](o), the rise time to peak [K(+)](o) (t(1/2), 2.3 +/- 0.5 s) as well as the recovery to baseline [K(+)](o) (t(1/2), 15.6 +/- 1.5 s) were slowed in AQP4 -/- mice compared to WT mice (t(1/2), 0.5 +/- 0.1 and 6.6 +/- 0.7 s, respectively). These results implicate AQP4 in the expression and termination of seizure activity and support the hypothesis that AQP4 is coupled to potassium homeostasis in vivo.

Roles of Aquaporins in Kidney Revealed by Transgenic Mice

Transgenic mouse models of aquaporin (AQP) deletion and mutation have been instructive in elucidating the role of AQPs in renal physiology. Mice lacking AQP1 are unable to concentrate their urine because of low water permeability in the proximal tubule, thin descending limb of Henle, and outer medullary descending vasa recta, resulting in defective near-isosmolar fluid absorption in the proximal tubule and defective countercurrent multiplication. Mice lacking functional AQP2, AQP3, or AQP4 manifest various degrees of nephrogenic diabetes insipidus resulting from reduced collecting duct water permeability. Mice lacking AQP7 and AQP8 can concentrate their urine fully, although AQP7 null mice manifest an interesting defect in glycerol reabsorption. Two unexpected renal phenotypes of AQP null mice have been discovered recently, including defective proximal tubule cell migration in AQP1 deficiency, and cystic renal disease in AQP11 deficiency. AQPs thus are important in several aspects of the urinary concentrating mechanism and in functions unrelated to tubular fluid transport. The mouse phenotype data suggest the renal AQPs as targets for the development of aquaretics and potentially for therapy of cystic renal disease and acute renal injury.

Three distinct roles of aquaporin-4 in brain function revealed by knockout mice

Aquaporin-4 (AQP4) is expressed in astrocytes throughout the central nervous system, particularly at the blood-brain and brain-cerebrospinal fluid barriers. Phenotype analysis of transgenic mice lacking AQP4 has provided compelling evidence for involvement of AQP4 in cerebral water balance, astrocyte migration, and neural signal transduction. AQP4-null mice have reduced brain swelling and improved neurological outcome in models of (cellular) cytotoxic cerebral edema including water intoxication, focal cerebral ischemia, and bacterial meningitis. However, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema including cortical freeze-injury, brain tumor, brain abscess and hydrocephalus, probably due to impaired AQP4-dependent brain water clearance. AQP4 deficiency or knock-down slows astrocyte migration in response to a chemotactic stimulus in vitro, and AQP4 deletion impairs glial scar progression following injury in vivo. AQP4-null mice also manifest reduced sound- and light-evoked potentials, and increased threshold and prolonged duration of induced seizures. Impaired K+ reuptake by astrocytes in AQP4 deficiency may account for the neural signal transduction phenotype. Based on these findings, we propose modulation of AQP4 expression or function as a novel therapeutic strategy for a variety of cerebral disorders including stroke, tumor, infection, hydrocephalus, epilepsy, and traumatic brain injury.

Aquaporin 4 is increased in association with human immunodeficiency virus dementia: implications for disease pathogenesis

Changes in astrocyte shape and function are known to occur in association with human immunodeficiency virus (HIV) dementia (HIVD). However, the causes and consequences of such changes are not completely understood. In vitro data suggest that changes in the expression of aquaporin 4 (AQP4), the aquaporin subtype expressed by astrocytes, can significantly influence cell shape and physiology. In the present study, the authors therefore investigated the possibility that AQP4 levels may be altered in HIVD. Using Western blot, the authors show that immunoreactivity for AQP4 is elevated in brain homogenates from the mid frontal gyrus of patients who died with HIVD (P < .005 HIV seronegative versus HIVD). Of interest, a significant increase was also observed in homogenates from HIV-infected individuals without dementia (P < .05 HIV seronegative versus neurologically normal HIV seropositive). In the present study the authors also examined the stimulated expression of AQP4 in cultured cells. Previous in vitro studies have shown that AQP4 expression may be increased by stimuli that induce cytoskeletal changes and/or the activation of p38 mitogen-activated protein (MAP) kinase. The authors therefore focused on tumor necrosis factor (TNF)-alpha, which has been linked to p38 MAP kinase activation, and thrombin, which may also induce changes in the actin cytoskeleton. Both may be elevated with HIVD. Again using Western blot, the authors show an increase in both AQP4 and phosphorylated p38 MAP kinase in homogenates from TNF-alpha- and thrombin-stimulated organotypic cerebellar and spinal cord cultures. Together, these studies suggest that AQP4 expression may be altered in HIVD and/or in response to exogenous proteinases. Additional studies may be warranted to determine whether altered AQP4 expression represents a protective and/or maladaptive response to central nervous system (CNS) inflammation.

Impaired pain sensation in mice lacking Aquaporin-1 water channels

Aquaporin-1 (AQP1), a membrane water channel, is expressed in choroid plexus where it contributes to cerebrospinal fluid production. Here, we show that AQP1 is also expressed in the dorsal horn of the spinal cord and the trigeminal nucleus caudalis, regions that process pain information. Within the dorsal root and trigeminal sensory ganglia, AQP1 is concentrated in small diameter cell bodies, most of which give rise to unmyelinated C-fibers. To study the role of AQP1 in pain signaling, we compared acute pain responses in wild-type mice and in mice lacking AQP1. AQP1(-/-) mice had reduced responsiveness to thermal and capsaicin chemical stimuli, but not to mechanical stimuli or formalin. These results provide evidence for AQP1 expression in nociceptive neurons and suggest that AQP1 may play a role in pain signal transduction.

Sulforaphane enhances aquaporin-4 expression and decreases cerebral edema following traumatic brain injury

Brain edema, the infiltration and accumulation of excess fluid causing an increase in brain tissue volume, often leads to a rise in intracranial pressure and is a key contributor to the morbidity and mortality associated with traumatic brain injury (TBI). The cellular and molecular mechanisms contributing to the development/resolution of TBI-associated brain edema are poorly understood. Aquaporin-4 (AQP4) water channel is expressed at high levels in brain astrocytes, and the bidirectional transport of water through these channels is critical for the maintenance of brain water homeostasis. By using a rodent injury model, we show that TBI decreased AQP4 level in the injury core and modestly increased it in the penumbra region surrounding the core. Postinjury administration of sulforaphane (SUL), an isothiocyanate present in abundance in cruciferous vegetables such as broccoli, attenuated AQP4 loss in the injury core and further increased AQP4 levels in the penumbra region compared with injured animals receiving vehicle. These increases in AQP4 levels were accompanied by a significant reduction in brain edema (assessed by percentage water content) at 3 days postinjury. These findings suggest that the reduction of brain edema in response to SUL administration could be due, in part, to water clearance by AQP4 from the injured brain.

Increased seizure duration in mice lacking aquaporin-4 water channels

Aquaporins are intrinsic membrane proteins involved in water transport in fluid-transporting tissues. In the brain, aquaporin-4 (AQP4) is expressed widely by glial cells, but its function is unclear. Extensive basic and clinical studies indicate that osmolarity affects seizure susceptibility, and in our previous studies we found that AQP4 -/- mice have an elevated seizure threshold in response to the chemoconvulsant pentylenetetrazol. In this study, we examined the seizure phenotype of AQP4 -/- mice in greater detail using in vivo electroencephalographic recording. AQP4 -/- mice were found to have dramatically longer stimulation-evoked seizures following hippocampal stimulation as well as a higher seizure threshold. These results implicate AQP4 in water and potassium regulation associated with neuronal activity and seizures.

Diffusion tensor imaging in medial temporal lobe epilepsy with hippocampal sclerosis

Interictal diffusion imaging studies in patients with medial temporal lobe epilepsy (MTLE) accompanied by hippocampal sclerosis (HS) have shown an increased diffusivity in the epileptogenic hippocampus. In this study, we wanted to explore the whole brain in order to determine if MTLE could have an impact on the organization and the architecture of a large cerebral network and to identify clinical factors that could mediate diffusion abnormalities. Diffusion tensor imaging (DTI) and statistical parametric mapping of the entire brain were performed in 35 well-defined MTLE patients and in 36 healthy volunteers. SPM analyses identified three abnormal areas: an increased diffusivity was detected in the epileptic hippocampus and the ipsilateral temporal structures associated with a decreased anisotropy along the temporal lobe, a decreased diffusivity was found in the contralateral non-sclerotic hippocampus, the amygdala, and the temporal pole, and finally, a decreased anisotropy was noted ipsilaterally in posterior extratemporal regions. Duration of epilepsy, age at onset, and the frequency of generalized tonic-clonic seizures or partial complex seizures did not correlate with the presence of diffusion abnormalities. Region of interest analysis in the hippocampus/parahippocampus demonstrated a correlation between lower ipsilateral diffusivity values and occurrence of epigastric aura and between higher anisotropy values in both hemispheres and history of febrile seizures. In conclusion, this study showed that diffusion abnormalities are not restricted to the pathologic hippocampus and involve a larger network. This pattern may indirectly reflect the epileptogenic network and may be interpreted as a cause or a consequence of epilepsy.

K+ waves in brain cortex visualized using a long-wavelength K+-sensing fluorescent indicator

We synthesized a water-soluble, long-wavelength K(+) sensor, TAC-Red, consisting of triazacryptand coupled to 3,6-bis(dimethylamino)xanthylium, whose fluorescence increased 14-fold at 0-50 mM K(+) with K(+)-to-Na(+) selectivity >30. We visualized K(+) waves in TAC-Red-stained brain cortex in mice during spreading depression, with velocity 4.4 +/- 0.5 mm/min, and K(+) release and reuptake half-times (t(1/2)) of 12 +/- 2 and 32 +/- 4 s, respectively. Aquaporin-4 (AQP4) deletion slowed K(+) reuptake about twofold, suggesting AQP4-dependent K(+) uptake by astroglia.

Neuronal versus glial cell swelling in the ischaemic retina

Under normal conditions, the pigment epithelium dehydrates the outer retina while Müller glial cells mediate the rapid water transport within the inner retina. Gliotic alterations of Müller cells may be implicated in the development of oedema in the post-ischaemic retina. Here, we suggest a mechanism of Müller cell-supported neuronal cell swelling and apoptosis in the ischaemic retina. During ischaemia, over-excitation of ionotropic glutamate receptors leads to neuronal cell depolarization that causes excess Ca(2+) influx into the cells, and to activation of the apoptosis machinery. The ion fluxes into the retinal neurons are associated with water movements that are mediated by aquaporin-4 water channels expressed by Müller cells and result in neuronal cell swelling. After reperfusion, the glial cells may swell due to the down-regulation of their K(+) conductance, which results in intracellular K(+) overload and water movements from the blood and vitreous into the cells. An inhibition of the glial cell-mediated water movements during ischaemic episodes should reduce the ion shifts at the neuronal synapses, resulting in decreased neuronal cell swelling and apoptosis. An inhibition of the water movements in the post-ischaemic phase may prevent cytotoxic Müller cell swelling but may impair the fluid clearance from retinal tissue in the presence of vasogenic oedema. Thus, pharmacological modification of the ion and fluid clearance functions of Müller cells may become a novel way to resolve both cytotoxic and vasogenic oedema in the retina.

More than just water channels: unexpected cellular roles of aquaporins

Aquaporins (AQPs) are membrane proteins that transport water and, in some cases, also small solutes such as glycerol. AQPs are expressed in many fluid-transporting tissues, such as kidney tubules and glandular epithelia, as well as in non-fluid-transporting tissues, such as epidermis, adipose tissue and astroglia. Their classical role in facilitating trans-epithelial fluid transport is well understood, as in the urinary concentrating mechanism and gland fluid secretion. AQPs are also involved in swelling of tissues under stress, as in the injured cornea and the brain in stroke, tumor and infection. Recent analysis of AQP-knockout mice has revealed unexpected cellular roles of AQPs. AQPs facilitate cell migration, as manifested by reduced tumor angiogenesis in AQP1-knockout mice, by a mechanism that might involve facilitated water transport in lamellipodia of migrating cells. AQPs that transport both glycerol and water regulate glycerol content in epidermis and fat, and consequently skin hydration/biosynthesis and fat metabolism. AQPs might also be involved in neural signal transduction, cell volume regulation and organellar physiology. The many roles of AQPs could be exploited for clinical benefit; for example, treatments that modulate AQP expression/function could be used as diuretics, and in the treatment of brain swelling, glaucoma, epilepsy, obesity and cancer.

Bilateral, vascular and perivascular glial upregulation of heat shock protein-27 after repeated epileptic seizures

Heat shock protein-27 (HSP-27) is an inducible stress response protein. It inhibits apoptotic cell death and is a reliable marker for oxidative stress. We studied the induction of HSP-27 in rat brains on days 1, 4 and 14 after repeated, pentylenetetrazole (PTZ)-induced seizures using immunohistochemisty. Saline treated control rats showed no induction of HSP-27. HSP-27 reactive astrocytes were rarely seen 1 or 4 days after PTZ injection. When present, single astrocytes were located in the cortex and/or the hippocampus. After 14 days PTZ treatment, a bilateral distribution of HSP-27 immunoreactive glia was present in piriform and entorhinal cortices and in the dentate gyrus of most brains. Rats with most intense HSP-27 upregulation showed HSP-27 in amygdala and thalamic nuclei. Astrocytes associated with blood vessels presented strongest HSP-27 staining, but did not show upregulation of gial fibrillary acidic protein and none responded with HSP-47 expression. Additionally, HSP-27 immunoreactivity increased in the endothelial cells of blood vessels in the affected brain regions, although no neuronal induction occurred. Contrastingly, a subconvulsive dose of the glutamine synthetase inhibitor L-methionine sulfoxime, which acts directly on astrocytes, resulted in a rapid, homogeneous astrocyte-specific HSP-27 upregulation within 24 h. Thus, repeated PTZ-induced seizure activity elicits a focal "heat shock" response in endothelial cells and astrocytes of selected cerebral regions indicating that expression of HSP-27 occurred in a seizure-dependent manner within the affected cerebral circuitries. Therefore, this PTZ-model of repeated seizure activity exhibited a cortical pattern of HSP-27 expression which is most comparable to that known from patients with epilepsy.

In vivo measurement of brain extracellular space diffusion by cortical surface photobleaching

Molecular diffusion in the brain extracellular space (ECS) is an important determinant of neural function. We developed a brain surface photobleaching method to measure the diffusion of fluorescently labeled macromolecules in the ECS of the cerebral cortex. The ECS in mouse brain was labeled by exposure of the intact dura to fluorescein-dextrans (M(r) 4, 70, and 500 kDa). Fluorescein-dextran diffusion, detected by fluorescence recovery after laser-induced cortical photobleaching using confocal optics, was slowed approximately threefold in the brain ECS relative to solution. Cytotoxic brain edema (produced by water intoxication) or seizure activity (produced by convulsants) slowed diffusion by >10-fold and created dead-space microdomains in which free diffusion was prevented. The hindrance to diffusion was greater for the larger fluorescein-dextrans. Interestingly, slowed ECS diffusion preceded electroencephalographic seizure activity. In contrast to the slowed diffusion produced by brain edema and seizure activity, diffusion in the ECS was faster in mice lacking aquaporin-4 (AQP4), an astroglial water channel that facilitates fluid movement between cells and the ECS. Our results establish a minimally invasive method to quantify diffusion in the brain ECS in vivo, revealing stimulus-induced changes in molecular diffusion in the ECS with unprecedented spatial and temporal resolution. The in vivo mouse data provide evidence for: (1) dead-space ECS microdomains after brain swelling; (2) slowed molecular diffusion in the ECS as an early predictor of impending seizure activity; and (3) a novel role for AQP4 as a regulator of brain ECS.

Phenotype analysis of aquaporin-8 null mice

Aquaporin-8 (AQP8) is a water-transporting protein expressed in organs of the mammalian gastrointestinal tract (salivary gland, liver, pancreas, small intestine, and colon) and in the testes, heart, kidney, and airways. We studied the phenotype of AQP8-null mice, and mice lacking AQP8, together with AQP1 or AQP5. AQP8-knockout mice lacked detectable AQP8 transcript and protein, and had reduced water permeability in plasma membranes from testes. Breeding of AQP8 heterozygous mice yielded AQP8-null mice, whose number, survival, and growth were not different from those of wild-type mice. Organ weight and serum/urine chemistries were similar in wild-type and AQP8-null mice, except for increased testicular weight in the null mice (4.8 ± 0.7 vs. 7.3 ± 0.3 mg/g body wt). Urinary concentrating ability in AQP8-null mice was unimpaired as assessed by urine osmolality (3,590 ± 360 mosmol/kgH2O) and weight loss (22 ± 2%) after 36-h water deprivation; urinary concentrating ability was similarly impaired in AQP1-null mice vs. AQP8/AQP1 double-knockout mice. Agonist-driven fluid secretion in salivary gland was not different in AQP8 vs. wild-type mice (∼1 μl·min−1·g body wt−1) or in AQP5-null mice vs. AQP8/AQP5 double-knockout mice. Closed intestinal loop measurements in vivo indicated unimpaired osmotically driven water transport, active fluid absorption, and cholera toxin-driven fluid secretion in AQP8-null mice. After 21 days on a 50% fat diet, wild-type and AQP8-null mice had similar weight gain (∼15 g), with no evidence of steatorrhea or abnormalities in blood chemistries, except for mild hypertriglyceridemia in the null mice. The mild phenotype of AQP8-null mice was surprising in view of the multiple phenotype abnormalities found in mouse models of AQP1–5 deficiency.

Aquaporin-4 in the central nervous system: cellular and subcellular distribution and coexpression with KIR4.1

Aquaporin-4 (AQP4) is the predominant water channel in the neuropil of the central nervous system. It is expressed primarily in astrocytes, but also occurs in ependymocytes and endothelial cells. A striking feature of AQP4 expression is its polarized distribution in brain astrocytes and retinal Muller cells. Thus, immunogold analyses have revealed an enrichment of AQP4 in endfeet membranes in contact with brain microvessels or subarachnoidal space and a low but significant concentration in non-endfeet membranes, including those astrocyte membranes that ensheath glutamate synapses. The subcellular compartmentation of AQP4 mimics that of the potassium channel Kir4.1, which is implicated in spatial buffering of K(+). We propose that AQP4 works in concert with Kir4.1 and the electrogenic bicarbonate transporter NBC and that water flux through AQP4 contributes to the activity dependent volume changes of the extracellular space. Such volume changes are important as they affect the extracellular solute concentrations and electrical fields, and hence neuronal excitability. We conclude that AQP4-mediated water flux represents an integral element of brain volume and ion homeostasis.

New insights into water transport and edema in the central nervous system from phenotype analysis of aquaporin-4 null mice

Aquaporin-4 (AQP4) is the major water channel in the CNS. Its expression at fluid-tissue barriers (blood-brain and brain-cerebrospinal fluid barriers) throughout the brain and spinal cord suggests a role in water transport under normal and pathological conditions. Phenotype studies of transgenic mice lacking AQP4 have provided evidence for a role of AQP4 in cerebral water balance and neural signal transduction. Primary cultures of astrocytes from AQP4-null mice have greatly reduced osmotic water permeability compared with wild-type astrocytes, indicating that AQP4 is the principal water channel in these cells. AQP4-null mice have reduced brain swelling and improved neurological outcome following water intoxication and focal cerebral ischemia, establishing a role of AQP4 in the development of cytotoxic (cellular) cerebral edema. In contrast, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema caused by freeze-injury and brain tumor, probably due to impaired AQP4-dependent brain water clearance. AQP4-null mice also have markedly reduced acoustic brainstem response potentials and significantly increased seizure threshold in response to chemical convulsants, implicating AQP4 in modulation of neural signal transduction. Pharmacological modulation of AQP4 function may thus provide a novel therapeutic strategy for the treatment of stroke, tumor-associated edema, epilepsy, traumatic brain injury, and other disorders of the CNS associated with altered brain water balance.

Aquaporin-4 facilitates reabsorption of excess fluid in vasogenic brain edema

Aquaporin-4 (AQP4) is the major water channel in the brain, expressed predominantly in astroglial cell membranes. Initial studies in AQP4-deficient mice showed reduced cellular brain edema following water intoxication and ischemic stroke. We hypothesized that AQP4 deletion would have the opposite effect (increased brain swelling) in vasogenic (noncellular) edema because of impaired removal of excess brain water through glial limitans and ependymal barriers. In support of this hypothesis, we found higher intracranial pressure (ICP, 52+/-6 vs. 26+/-3 cm H2O) and brain water content (81.2+/-0.1 vs. 80.4+/-0.1%) in AQP4-deficient mice after continuous intraparenchymal fluid infusion. In a freeze-injury model of vasogenic brain edema, AQP4-deficient mice had remarkably worse clinical outcome, higher ICP (22+/-4 vs. 9+/-1 cm H2O), and greater brain water content (80.9+/-0.1 vs. 79.4+/-0.1%). In a brain tumor edema model involving stereotactic implantation of melanoma cells, tumor growth was comparable in wild-type and AQP4-deficient mice. However, AQP4-deficient mice had higher ICP (39+/-4 vs. 19+/-5 cm H2O at seven days postimplantation) and corresponding accelerated neurological deterioration. Thus, AQP4-mediated transcellular water movement is crucial for fluid clearance in vasogenic brain edema, suggesting AQP4 activation and/or up-regulation as a novel therapeutic option in vasogenic brain edema.

Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: Aquaporins in the Brain

Water homeostasis of the brain is essential for its neuronal activity. Changes in water content in the intra- and extra-cellular space affect ionic concentrations and therefore modify neuronal activity. Aquaporin (AQP) water channels may have a central role in keeping water homeostasis in the brain. Among AQP subtypes cloned in mammalian, only AQP1, AQP4, and AQP9 were identified in the brain. Changes in AQP expression may be correlated with edema formation of the brain. In this review, we describe the physiological function of AQPs and the regulatory mechanism of their expression in the brain.

Anchoring of aquaporin-4 in brain: Molecular mechanisms and implications for the physiology and pathophysiology of water transport

Astrocytes show an enrichment of aquaporin-4 (AQP4) in those parts of the plasma membrane that are apposed to pial or perivascular basal laminae. This observation begged the following questions: 1, What are the molecular mechanisms that are responsible for the site specific anchoring of AQP4? 2, What are the physiological and pathophysiological roles of the AQP4 pools at these specialized membrane domains? Recent studies suggest that the site specific anchoring depends on the dystrophin complex. Further, alpha-syntrophin (a member of the dystrophin complex) is required to maintain a polarized expression of AQP4 in the perivascular membranes. Hence transgenic mice deficient in alpha-syntrophin provided a model where the perivascular pool of AQP4 could be removed for assessment of its functional roles. Data suggest that the perivascular pool of AQP4 plays a role in edema formation and that this pool (through its serial coupling with the AQP4 pools in other astrocyte membranes) is involved in K(+) siphoning. In the cerebral cortex, the astrocyte membrane domain contacting the pial basal lamina differs from the perivascular membrane domain in regard to the mechanisms for AQP anchoring. Thus deletion of alpha-syntrophin causes only a 50% loss of AQP4 from the former membrane (compared with a 90% loss in the latter), pointing to the existence of additional anchoring proteins. We will also discuss the subcellular distribution and anchoring of AQP4 in the other cell types that express this protein: endothelial cells, ependymal cells, and the specialized astrocytes of the osmosensitive organs.

From finch to fish to man: Role of aquaporins in body fluid and brain water regulation

Charles Darwin, in his Origin of the Species, noted that different species of finches on the Galapagos Islands had adapted their beak size based on where they sought their food. Homer Smith, in his book From Fish to Philosopher, discussed the evolution of the nephron from a single conduit in salt water vertebrates, to nephrons with large glomerular capillaries and proximal and distal tubules in fresh water vertebrates, to smaller glomerular capillaries in amphibians, to nephrons with loops of Henle to allow for urinary concentration and dilution in mammals. The kidney with its million nephrons has emerged as the vital organ for regulating body fluid composition and volume. With the recent discovery of aquaporin water channels, our understanding of volume regulation has been greatly enhanced. This article reviews current knowledge regarding: 1) the unifying hypothesis of body fluid volume regulation; 2) brain aquaporins and their role in pathophysiologic states; and 3) function and regulation of renal aquaporins in the syndrome of inappropriate antidiuretic hormone secretion (SIADH).